A modification of the numerical aperture of the illuminating or imaging optical system causes a change in the resolution achievable with the optical imaging system and a change in the contrast of the image generated using the optical imaging system. In microscopes in particular, in order to modify the numerical aperture a diaphragm, or aperture device, whose diameter is variably adjustable is arranged in the illumination beam path. At a maximum aperture diameter, the entire numerical aperture of a condenser and a microscope objective is illuminated, furnishing an image having maximum resolution but low contrast. If, on the other hand, the aperture diameter is decreased or minimized, the optical imaging system then furnishes less resolution but increased contrast.
Upon a change in resolution and contrast by means of the aperture provided in the illumination beam path, however, the light flux through the illumination beam path and into the microscope objective is also modified, so that the brightness of the image additionally changes. A decrease in the diameter of the aperture in the illumination beam path therefore results not only in an image having lower resolution and higher contrast, but also in a darker image.
Two different illumination modes are usually utilized in a microscope, namely transmitted-light and incident-light illumination. The aperture device is usually arranged in the illumination beam path, and with it the numerical aperture of the illumination beam path can then be modified. The illumination beam path extends from the light source to the specimen. In principle, however, the aperture device can also be arranged in the imaging beam path. Some microscopes, for example, have a capability for internal numerical aperture adjustment by way of a built-in iris diaphragm, so that in this case the aperture device is arranged in the imaging beam path. The imaging beam path extends from the specimen to the microscope user's eye, or to the image plane of a camera onto which the specimen is imaged. The discussion hereinafter will be based principally on a microscope which has an aperture device in the illumination beam path.
Also conceivable, in principle, is a microscope configuration in which an aperture device is arranged in the imaging beam path or in the imaging and the illumination beam path, so that the subject matter of the present invention is likewise analogously applicable to these configurations. The aperture device modifies the numerical aperture of the beam path in which it is arranged, and thus the imaging resolution and the contrast in the image.
A modification of the settings regarding the resolution and contrast of the microscope is overlain by a change in the brightness of the microscope image. Especially in quantitative microscopy, in which comparative measurements of different specimens need to be made, this kind of brightness change in the microscope image furnishes distorted results. Ultimately, these correlations very considerably complicate adjustment of these operating parameters of a microscope.
With incandescent or halogen lamps in particular, however, the color temperature or spectral intensity distribution of the light emitted by the respective light source changes if the electrical power delivered to the light source is modified. This is because incandescent lamps exhibit light emission behavior similar to that of a black body, so that when the delivered electrical power is reduced, the color temperature of the spectrum emitted by the incandescent lamp shifts from the blue spectral region toward the red spectral region; this is also referred to generally as a “red shift.” This causes a change in the perceived color of the specimen image, although with the microscope according to the present invention there is almost no change in the brightness of the specimen image.